This invention relates to a switching circuit, in particular for switching electrical bipolar power to a load in an automatic external defibrillator.
Automatic external defibrillators (AEDs) commonly deliver electrotherapy by the bipolar transfer of energy into a patient. Appropriate therapy demands a significantly high voltage, in excess of 1000 volts at a current in excess of 30 amps. This puts exceedingly stringent requirements on the design of the switching circuit used to deliver such therapy, not only on the components needed to deliver such energy with integrity and reliability, but also on the safety of such devices.
A known switching circuit is an H-bridge, so-called from its typical graphical representation in the form of an “H”. An H-bridge can be used in many situations where it is desired to reverse the direction of current through a load, for example to drive a reversible DC electric motor or, in the case of an AED, to reverse the direction of current through a patient's torso. In general, an H-bridge has four solid state or mechanical switching devices arranged respectively in the four “legs” of the H which are switched on in alternate diagonal pairs to deliver a current to a load first in one direction and then the other from a voltage source.
An example of an H-bridge 10 used for current switching in an AED is shown in FIG. 1, where the load is the patient and the voltage, derived from a capacitor V, is applied via electrodes applied to the patient's chest. In the H-bridge 10 the switching devices S11 and S12 in the ‘high’ (i.e. non-grounded) legs of the H-bridge are silicon controlled rectifiers (SCRs) and the switching devices S13 and S14 in the ‘low’ legs of the H-bridge are insulated gate bipolar transistors (IGBTs). An SCR is triggered by a pulse of a minimum duration on its gate at a specified voltage above that of its cathode but it is susceptible to spontaneous switch-on if the rate of increase of voltage across it exceeds a specified limit. It is switched off by the reduction of the current through it below a specified level. An IGBT is switched on by, and for the duration of, a voltage applied to its gate greater than a specified level above its emitter voltage. It is switched off by the removal of this trigger voltage. The operation of the H-bridge 10 is as follows.
The first part of the bipolar delivery into the load is initiated by switching on the IGBT S13. This is achieved by applying and maintaining an external trigger signal Control 3 to its gate. The capacitor V is then charged (using an external charging circuit, not shown) from zero to the required voltage for delivery into the load at the required current. It is necessary that the capacitor V is not pre-charged prior to S13 being switched on since any voltage appearing instantaneously across an SCR, such as the SCR S11, may spontaneously trigger the SCR due to the rate of increase in voltage across it. When the required voltage on the capacitor V is reached, the diagonally opposite SCR S12 is switched on by applying a pulse Control 2 to its gate via a coupling transformer (not shown). When both switches S12 and S13 are conducting, current passes through the load in one direction. At a chosen time, the IGBT S13 is switched off by the removal of the trigger signal Control 3 on its gate. This removes the current through the SCR S12 causing it to switch off, thereby disconnecting the load from the voltage supply V.
The second part of the bipolar delivery into the load is initiated by switching on the IGBT S14 by applying and maintaining an external trigger signal Control 4 to its gate. Immediately afterwards, the diagonally opposite SCR S11 is switched on by a pulse Control 1 to its gate via a coupling transformer. Now that both switches S11 and S14 are conducting, current passes through the load in the opposite direction to that during the first part of the bipolar discharge. The cycle ends when either the IGBT S14 is switched off, thereby removing the current through the SCR S11 which consequently switches off, or by the discharge of the primary voltage source V to a point at which the SCR S11 cannot support the reduced current flow in the load.
A limitation of the known H-bridge is the complexity of the circuitry needed to drive the SCRs S11 and S12. In both cases, since the voltage on the cathodes of the SCRs can rise to the same potential as the voltage across the capacitor V, the gates must be decoupled with transformers. from external circuits to inject the pulses needed for switching them on. Further, considerable additional circuitry is required to implement hardware interlocking circuits to ensure the safety and integrity of the operation of the bridge.
U.S. Pat. No. 6,996,436 teaches the replacement of one of the SCRs by an uncontrolled solid-state device (USD) having Shockley device characteristics. This eliminates the necessity for one transformer coupling and, since the USD switches as a direct consequence of the action of the IGBTs, the hardware interlocking requirements for integrity are also reduced. However, a further limitation is imposed in that due to its Shockley characteristics the USD cannot switch below a certain threshold voltage. Therefore, when used in an automatic external defibrillator, it will not be possible to deliver energy below a level defined by the lowest voltage level at which the USD can operate.
It is an object of this invention to provide an external defibrillator having a switching circuit which avoids or mitigates these disadvantages.
Accordingly, the present invention provides an external defibrillator comprising an H-bridge for delivering bipolar electrotherapy to a patient, the H-bridge having a respective solid state switching device in each of its four legs, the switching device in at least one of the high legs of the H-bridge including an SCR, the H-bridge further comprising a control circuit which automatically generates a voltage spike to turn on the SCR in response to a voltage change across the SCR which occurs when the switching device in the diagonally opposite low leg turns on.